The text here, as corrected in 𝓞, is the original reading: namely, “because of the tradition of your fathers which leads you away into a belief of things which are not so”. Oliver Cowdery initially wrote traditions in 𝓞, but then he erased the plural s (an immediate correction), thus giving “because of the tradition which leads you away” (which is perfectly grammatical). Oliver also initially wrote “into a belief of things which is not so”, which he virtually immediately corrected to “into a belief of things which are not so” (his crossout of the is and supralinear insertion of the are show no change in the level of ink flow). The initial is may have been influenced by the preceding singular belief; the antecedent for the relative pronoun which is, of course, the immediately preceding plural things.
Subsequent history of this passage led to textual difficulties. When he copied from 𝓞 into 𝓟, Oliver Cowdery changed the verb leads to lead, probably because he was influenced by the immediately preceding plural noun fathers, even though the relative clause “which lead(s) you away” refers to the traditions, not the fathers. Yet such disagreement between subject and verb is frequently found in the original text. (See, for instance, the discussion under 1 Nephi 4:4; for general discussion, see under subject-verb agreement in volume 3.) Finally, when Joseph
Smith came to edit this passage for the 1840 edition, he removed the subject-verb disagreement by changing tradition into a plural. The 1852 LDS edition made the same change in the LDS text, perhaps independently or by reference to the 1840 edition. The 1908 RLDS edition, on the other hand, restored the original tradition since 𝓟 read that way (yet that edition maintained the plural verb form lead, also the reading in 𝓟, even though the resulting reading led to subject-verb disagreement). The critical text will, of course, restore the corrected reading in 𝓞.
For further discussion regarding subject-verb agreement for the noun tradition(s) as well as other changes involving the grammatical number for tradition(s), see under Mosiah 1:5.
Summary: Restore in Alma 30:16 the corrected reading in 𝓞: “because of the tradition of your fathers which leads you away into a belief of things which are not so”; this reading, most likely the original reading, is grammatically correct according to the standard rules of subject-verb agreement.